The present invention relates to a diesel engine with a turbocharge and particularly to an exhaust gas recirculation system in such engine.
Such an exhaust gas recirculation system has been used in order to cope with the exhaust gas discharged from a diesel engine and particularly to decrease the amount of oxides of nitrogen (NO.sub.x) discharged from the engine by suppressing an increase in the temperature of the engine.
One conventional exhaust gas recirculation system of the above type is shown in FIG. 1 of the drawings. In this system, generally designated by reference numeral 10, when a diesel engine 12 is operating at a relatively low speed, the flow rate of air supplied from a compressor 14 of a supercharger 16 through an inlet pipe 18 to the diesel engine 12 is low and thus the supercharged pressure from the compressor 14 is low. This supercharged pressure is supplied through a pipe 20 to a chamber 22 of a valve actuating device 24 formed by partitioning a housing 26 with a diaphragm 28. The exhaust gas pressure of the engine 12 is introduced through an exhaust pipe 30 to a turbine 32 of the supercharger 16 and through a recirculation passage 34 to the other chamber 36 in the housing 26. If the pressure, designated by P.sub.1, inside the chamber 22 is lower than the pressure, designated by P.sub.2, inside the chamber 36, a valve rod 38 connected at one end to the diaphragm 28 is moved by a return spring 40 within the chamber 36 and the pressure P.sub.2 to close a valve member 42 formed on the rod 38 against an outlet 44 of a valve casing 46 constituting together with the valve rod 38 a valve 41 thereby reducing the flow rate Fg of exhaust gas flowing from the recirculation passage 34 through an inlet 48 and the valve casing 46 to the outlet 44 and hence recirculated through the recirculation passage 34 to an intake passage 50 to the compressor 14. If the exhaust gas pressure from the engine 12 increases and the flow of intake air to the engine 12 does not change, the pressure P.sub.2 at the inlet 48 will increase thereby initially increasing the amount of exhaust gas recirculated. However, the pressure P.sub.2 within the chamber 36 also increases, which causes the valve rod 38 to move in the direction of closing the valve member 42 against the outlet 44 thereby reducing F.sub.g. In the same way, when P.sub.2 decreases, the valve member 42 is moved away from the outlet 44 so as to increase F.sub.g. Reference numeral 33 denotes an exhaust passage from the turbine 32.
When the engine 12 operates at a relatively high speed, the flow of intake air to the engine 12 and the pressure in the intake pipe 18 increase. This causes the pressure P.sub.1 in the chamber 22 to increase compared with the pressure P.sub.2 in the chamber 36 with the result that the rod 38 is moved by the diaphragm 28 against the return spring 40 in the direction of moving the valve member 42 away from the outlet 44. Thus the flow F.sub.g of recirculated exhaust gas increases in proportion to the amount of intake air to the engine, irrespective of fluctuations in the exhaust gas pressure from the engine. According to this system, the amount of NO.sub.x contained in the exhaust gas is highly reduced. However, in order to improve the operation and output power of the engine, for economy of fuel consumption and to prevent the generation of smoke, the flow rate of recirculated exhaust gas should preferably be as low as possible.
When the engine load is relatively low, the amount of NO.sub.x produced is small and recirculation of exhaust gas is substantially unnecessary; it should rather be avoided in view of a possiblity of the engine's stopping. On the other hand, when the engine load is relatively high, the recirculation of exhaust gas should be avoided because high output power and low fuel consumption are required of the engine.